Generally, in a nonaqueous electrolyte based battery, a separator is composed of a porous membrane or nonwoven fabric having through-pores with a pore size of about several tens nm to several μm to prevent occurrence of a short circuit due to contact between positive and negative electrodes while enabling ion-conduction between positive and negative electrodes. However, use of a separator having pores causes problems such as occurrence of a short circuit due to growth of a dendrite and ingress of foreign matter, vulnerability to deformation such as bending or compression, and difficulty of attainment of both thickness reduction and strength maintenance.
Examples of the solution to these problems include solid electrolytes, which are classified broadly into inorganic solid electrolytes and organic solid electrolytes. The organic solid electrolytes are classified into polymer gel electrolytes and polymer solid electrolytes (intrinsic polyelectrolytes).
An inorganic solid electrolyte includes anionic lattice points and metal ions, and many inorganic electrolytes having a practical ion-conductivity have been reported (e.g. Japanese Patent Laid-open Publication No. 2014-13772). Those inorganic solid electrolytes have the advantage that they are incombustible and have high safety, and also have a wide potential window. On the other hand, those inorganic solid electrolytes are difficult to put into practical use because of the following disadvantages specific to inorganic solids, i.e. they easily suffer brittle fracture, and thus cannot follow a volume change of an electrode; and they cannot form a favorable interface with an electrode that is an aggregate of particles.
The organic solid electrolytes include polymer gel electrolytes obtained by making an electrolyte solution semi-solid with a polymer. Application of such polymer gel electrolytes to batteries was first reported by Feuillade et al. in 1975 (G. Feuillade, Ph. Perche, J. Appl. Electrochem., 5, 63 (1975)). Since then, various reports have been made (e.g. Japanese Patent Laid-open Publication No. 2008-159496) to date, and the polymer gel electrolytes have been put into practical use in lithium polymer batteries. However, those gel electrolytes are poor in substantial strength in batteries and, therefore, in most cases, a porous membrane has been used in combination for avoiding contact between positive and negative electrodes.
Studies on a polymer solid electrolyte were first reported in a paper by Wright published in 1973 (P. V. Wright, Br. Polm. J., 7. 319 (1975)) and, to date, many results have been reported mainly on polyether-based solid electrolytes (e.g. Japanese Patent Laid-open Publication No. 2007-103145). However, those polymer solid electrolytes still have lower conductivity as compared to electrolyte solutions, and are required to be further improved for practical use. Since the ion-conductivity is closely related to the segment motion of a polymer, studies have been conducted mainly on polymers having a low glass transition temperature due to softening, branching and molecular weight reduction of a polymer structure. On the other hand, however, those polymers also have a reduced elastic modulus and reduced heat resistance, resulting in impairment of a function of suppressing contact between positive and negative electrodes as in the case of gel electrolytes.
As described above, polymers having a high elastic modulus and high strength and heat resistance when formed into a membrane generally have a rigid polymer structure and, therefore, are considered unable to exhibit a high ion-conductivity, and thus have been rarely studied to date.
Films formed of an aromatic polymer, typically an aromatic polyamide (aramid), an aromatic polyimide or the like, are excellent in mechanical properties such as an elastic modulus and strength, and heat resistance and are, therefore, used in various applications such as magnetic recording media and circuit boards.
International Publication No. WO 95/31499 discloses an ion-conductive film obtained using an aromatic polyamide, and according to this document, a swollen gel is obtained by a method including substituting a contained solvent (or contained washing water) and an electrolyte solution in a film production process while using a rigid polymer that is impermeable to the electrolyte solution after the polymer is formed into a film. Japanese Patent Laid-open Publication No. H09-302115 discloses a composite membrane in which voids in an aromatic polyamide porous membrane are filled with a polymer solid electrolyte. The membrane is intended to solve problems associated with a porous membrane, i.e. a short circuit and vulnerability to deformation, and problems associated with a polymer solid electrolyte, i.e. low mechanical strength and heat resistance.
However, in the production method described in International Publication No. WO 95/31499, it is impossible to perform a heat treatment, and it is difficult to obtain a film having dimensional stability at a high temperature. In addition, in a nonaqueous electrolyte based battery such as a lithium ion battery, the amount of water in the battery is controlled in a ppm order and, therefore, practical use of this method involves many challenges.
In Japanese Patent Laid-open Publication No. H09-302115, sufficient mechanical strength and a practical ion-conductivity are not attained. Further, a polymer solid electrolyte in pores does not have high-temperature stability and, therefore, high heat resistance is not achieved.
As described above, an ion-permeable membrane having high heat resistance, strength and flexibility, and practical ion permeability has not been reported.
It could therefore be helpful to provide an ion-permeable membrane excellent in heat resistance, strength, flexibility and ion permeability, a battery electrolyte membrane obtained using the ion-permeable membrane, and an electrode composite.